Scientists in the UK and Switzerland say consumers should rethink how they interpret the sun protection factor (SPF) printed on sunscreen bottles.

Retailers have long used SPF to indicate how long sunscreen protected skin can endure sunlight without burning. Concerns that topical sunscreens do not provide the protection they claim are not new and arise from discrepancies between simulated and natural sunlight. Lab tests also assume consumers apply an even layer of sunscreen (2mg per cm2of exposed skin). In reality, this is in the region of 0.5–1.5mg cm–3, is far from uniform, and there are other factors, such as perspiration and rubbing, to consider.